1. Field of the Invention
The present invention is directed to an MR scanner of the type having a setting mechanism for patient-dependent control of the radio-frequency fields and magnetic fields for adherence to individual SAR and dB/dt limits.
2. Description of the Prior Art
Modern high-performance MR tomography systems are to a high degree no longer hardware-limited with respect to the radio-frequency or gradient pulses that can be applied to the patient. On the contrary, SW and HW monitors must be utilized in order to assure that SAR and dB/dt limits are adhered to. This in turn limits many applications which have an optimum performance at the upper edge of the range of the characteristic quantities.
For example, the SAR calculation determines the maximum radio-frequency energy that can be applied on the basis of rough assumptions (model of a number of cylinders) based on the size of the patient under examination. The size of the patient is estimated in a table from the age and sex entered by the operator. A simplified body model likewise forms the basis in the calculation of the dB/dt stimulation. A source of error in common to both methods as conventionally implemented is that the individual body geometry of the patient deviates from the model formation. In any case, a conservative estimate, with a limitation of the application resulting therefrom, must be made.
The abstract of Japanese Application 03 284 241 discloses a method for determining a specific absorption rate (SAR). First, a rough magnetic resonance image of an examination subject is produced with a fast imaging method. A circle is drawn around the part of the examination subject of interest in the magnetic resonance image. The radius of the circle is determined. The radius of this circle is introduced into an equation for determining a number of excitations per time unit with an SAR value below a prescribable value. Finally, a series of excitation pulses is set with the number of excitations per time unit that has been thus identified as a limit.
An object of the present invention is to equip an MR scanner of the type initially described so that a patient-dependent control of the radio-frequency fields and magnetic fields can ensues in a simple way, and wherein limit values that are maximally but exactly adapted to the patient can be determined for an improved radio-frequency and gradient pulse performance.
This object is inventively achieved in an MR scanner having an optical measuring device containing a distance sensor preferably arranged before the admission opening of the scanner above the patient table for detecting (identifying) the surface of a patient and for defining a setting signal for the setting mechanism.
The surface of the current patient thus can be measured with the assistance of a linearly or planarly resolving optical distance sensor in conjunction with a pulsed laser light source. CMOS sensors with an integration time of 30-40 nsec and a topical resolution of approximately 0.5 cm that have recently because commercially available and do not contain any mechanical parts whatsoever and are completely integrated on a chip. An optical measuring device constructed therewith can be very easily integrated in MR scanners, and the laser that is already provided for use in positioning the patient can be advantageously simultaneously employed as a light-emitting device for the surface measurement.
Due to the enormously high speed of such sensors, the measurement of the surface of the patient can be implemented completely during the displacement time of the patient table, with the identified surface information of the current patient representing the exact input (size, dimensions, specific details of the body region) for the calculation of individually adapted SAR and dB/dt limits.
Particularly when a targeted acquisition of details of the body region of the patient to be examined additionally ensues corresponding to the scanner program that has been set, it is possible to avoid application limitations due to limit values that are pessimistic because they are only imprecisely known, and to instead carry out the examination with the RF and gradient performance that is maximally possible for the current patient.